A nick or inclusion, in the radius of a lightening hole on the C-channel, was introduced at manufacture and compromised the structural integrity of the main rotor blade. This nick caused a stress riser and was the origin of a crack that propagated to the blade's lower skin, trailing edge, and onto the top skin perpendicular to the span of the blade, where the crack stopped near the blade spar. A recent inspection of the blade by company maintenance failed to detect any damage or crack; this suggests that crack propagation may develop rapidly and be difficult to detect. The blades were subjected to a variety of operations and a varying number of torque events that likely exceeded the estimated average number of high-stress manoeuvres in the flight spectrum determined by MDHI. Large differences in the count of torque events are likely because the type of operations affects the number of torque events and the recording of this number relies on pilots keeping an accurate count. Hence, MDHI rotor blades are subjected to a fatigue factor that may not be accurately tracked by operators.Analysis A nick or inclusion, in the radius of a lightening hole on the C-channel, was introduced at manufacture and compromised the structural integrity of the main rotor blade. This nick caused a stress riser and was the origin of a crack that propagated to the blade's lower skin, trailing edge, and onto the top skin perpendicular to the span of the blade, where the crack stopped near the blade spar. A recent inspection of the blade by company maintenance failed to detect any damage or crack; this suggests that crack propagation may develop rapidly and be difficult to detect. The blades were subjected to a variety of operations and a varying number of torque events that likely exceeded the estimated average number of high-stress manoeuvres in the flight spectrum determined by MDHI. Large differences in the count of torque events are likely because the type of operations affects the number of torque events and the recording of this number relies on pilots keeping an accurate count. Hence, MDHI rotor blades are subjected to a fatigue factor that may not be accurately tracked by operators. The main-rotor blade failed because its structural integrity was compromised by a manufacturing defect at the edge of a lightening hole in the C-channel, internal to the blade, located about three inches inboard of the data plate. The defect at the lightening hole resulted in a fatigue crack that propagated to the lower skin, rearward to the trailing edge, and around to the top skin going forward in the direction of the blade spar.FindingsastoCausesandContributingFactors The main-rotor blade failed because its structural integrity was compromised by a manufacturing defect at the edge of a lightening hole in the C-channel, internal to the blade, located about three inches inboard of the data plate. The defect at the lightening hole resulted in a fatigue crack that propagated to the lower skin, rearward to the trailing edge, and around to the top skin going forward in the direction of the blade spar. The type of operation widely affects the number of torque events, and the recording of this number relies on the pilot keeping an accurate count. Differences in this count are likely; hence, component fatigue is not accurately tracked. Compliance with service letters and service bulletins issued by the manufacturer is not mandatory. Therefore, some operators may not be accurately tracking the torque events encountered by the helicopter blades. The periodicity for the inspection of the main rotor blades may be inadequate and insufficient to find cracks that might occur before catastrophic failure. No finite torque event number is established for the main rotor blades.Findings as to Risk The type of operation widely affects the number of torque events, and the recording of this number relies on the pilot keeping an accurate count. Differences in this count are likely; hence, component fatigue is not accurately tracked. Compliance with service letters and service bulletins issued by the manufacturer is not mandatory. Therefore, some operators may not be accurately tracking the torque events encountered by the helicopter blades. The periodicity for the inspection of the main rotor blades may be inadequate and insufficient to find cracks that might occur before catastrophic failure. No finite torque event number is established for the main rotor blades. The US Federal Aviation Authority is working on a notice of proposed rulemaking to expedite an airworthiness directive to effect mandatory compliance with MD Helicopters Inc.'s (MDHI) Service Bulletin SB369D-201R1 and Torque Events Inspection/Assessment. Transport Canada noted that compliance with publications recommending the incorporation of modifications, the performance of inspections, or times between overhaul, are optional. However, aircraft owners have a duty to be aware of the contents of these publications and to evaluate the need for compliance in light of their own circumstances. Commercial operators should have a formalized process for conducting this evaluation as part of the evaluation program required by Canadian Aviation Regulation706. Notice of Proposed Amendments (NPAs) 2002-249 and 2002-250, which the Canadian Aviation Advisory Council (CARAC) has approved, will be published in the near future. These NPAs amend CAR PartVI, making operators' responsibilities in this matter clear. Transport Canada will continue to monitor this investigation and is considering taking unilateral action through an airworthiness directive, should the Federal Aviation Authority not mandate MDHI Mandatory Service BulletinSB369D-201R1. MDHI no longer manufactures main-rotor blades for 500-series helicopters. Under parts manufacturing approval, Helicopter Technologies Corporation (HTC) now assures the supply of these blades. HTC has incorporated an ultrasonic inspection as part of its manufacturing process to check for subsurface defects, voids, and cracks that, if undetected, may affect the structural integrity of the blades.Safety Action The US Federal Aviation Authority is working on a notice of proposed rulemaking to expedite an airworthiness directive to effect mandatory compliance with MD Helicopters Inc.'s (MDHI) Service Bulletin SB369D-201R1 and Torque Events Inspection/Assessment. Transport Canada noted that compliance with publications recommending the incorporation of modifications, the performance of inspections, or times between overhaul, are optional. However, aircraft owners have a duty to be aware of the contents of these publications and to evaluate the need for compliance in light of their own circumstances. Commercial operators should have a formalized process for conducting this evaluation as part of the evaluation program required by Canadian Aviation Regulation706. Notice of Proposed Amendments (NPAs) 2002-249 and 2002-250, which the Canadian Aviation Advisory Council (CARAC) has approved, will be published in the near future. These NPAs amend CAR PartVI, making operators' responsibilities in this matter clear. Transport Canada will continue to monitor this investigation and is considering taking unilateral action through an airworthiness directive, should the Federal Aviation Authority not mandate MDHI Mandatory Service BulletinSB369D-201R1. MDHI no longer manufactures main-rotor blades for 500-series helicopters. Under parts manufacturing approval, Helicopter Technologies Corporation (HTC) now assures the supply of these blades. HTC has incorporated an ultrasonic inspection as part of its manufacturing process to check for subsurface defects, voids, and cracks that, if undetected, may affect the structural integrity of the blades. A nick or inclusion, in the radius of a lightening hole on the C-channel, was introduced at manufacture and compromised the structural integrity of the main rotor blade. This nick caused a stress riser and was the origin of a crack that propagated to the blade's lower skin, trailing edge, and onto the top skin perpendicular to the span of the blade, where the crack stopped near the blade spar. A recent inspection of the blade by company maintenance failed to detect any damage or crack; this suggests that crack propagation may develop rapidly and be difficult to detect. The blades were subjected to a variety of operations and a varying number of torque events that likely exceeded the estimated average number of high-stress manoeuvres in the flight spectrum determined by MDHI. Large differences in the count of torque events are likely because the type of operations affects the number of torque events and the recording of this number relies on pilots keeping an accurate count. Hence, MDHI rotor blades are subjected to a fatigue factor that may not be accurately tracked by operators.Analysis A nick or inclusion, in the radius of a lightening hole on the C-channel, was introduced at manufacture and compromised the structural integrity of the main rotor blade. This nick caused a stress riser and was the origin of a crack that propagated to the blade's lower skin, trailing edge, and onto the top skin perpendicular to the span of the blade, where the crack stopped near the blade spar. A recent inspection of the blade by company maintenance failed to detect any damage or crack; this suggests that crack propagation may develop rapidly and be difficult to detect. The blades were subjected to a variety of operations and a varying number of torque events that likely exceeded the estimated average number of high-stress manoeuvres in the flight spectrum determined by MDHI. Large differences in the count of torque events are likely because the type of operations affects the number of torque events and the recording of this number relies on pilots keeping an accurate count. Hence, MDHI rotor blades are subjected to a fatigue factor that may not be accurately tracked by operators. The main-rotor blade failed because its structural integrity was compromised by a manufacturing defect at the edge of a lightening hole in the C-channel, internal to the blade, located about three inches inboard of the data plate. The defect at the lightening hole resulted in a fatigue crack that propagated to the lower skin, rearward to the trailing edge, and around to the top skin going forward in the direction of the blade spar.FindingsastoCausesandContributingFactors The main-rotor blade failed because its structural integrity was compromised by a manufacturing defect at the edge of a lightening hole in the C-channel, internal to the blade, located about three inches inboard of the data plate. The defect at the lightening hole resulted in a fatigue crack that propagated to the lower skin, rearward to the trailing edge, and around to the top skin going forward in the direction of the blade spar. The type of operation widely affects the number of torque events, and the recording of this number relies on the pilot keeping an accurate count. Differences in this count are likely; hence, component fatigue is not accurately tracked. Compliance with service letters and service bulletins issued by the manufacturer is not mandatory. Therefore, some operators may not be accurately tracking the torque events encountered by the helicopter blades. The periodicity for the inspection of the main rotor blades may be inadequate and insufficient to find cracks that might occur before catastrophic failure. No finite torque event number is established for the main rotor blades.Findings as to Risk The type of operation widely affects the number of torque events, and the recording of this number relies on the pilot keeping an accurate count. Differences in this count are likely; hence, component fatigue is not accurately tracked. Compliance with service letters and service bulletins issued by the manufacturer is not mandatory. Therefore, some operators may not be accurately tracking the torque events encountered by the helicopter blades. The periodicity for the inspection of the main rotor blades may be inadequate and insufficient to find cracks that might occur before catastrophic failure. No finite torque event number is established for the main rotor blades. The US Federal Aviation Authority is working on a notice of proposed rulemaking to expedite an airworthiness directive to effect mandatory compliance with MD Helicopters Inc.'s (MDHI) Service Bulletin SB369D-201R1 and Torque Events Inspection/Assessment. Transport Canada noted that compliance with publications recommending the incorporation of modifications, the performance of inspections, or times between overhaul, are optional. However, aircraft owners have a duty to be aware of the contents of these publications and to evaluate the need for compliance in light of their own circumstances. Commercial operators should have a formalized process for conducting this evaluation as part of the evaluation program required by Canadian Aviation Regulation706. Notice of Proposed Amendments (NPAs) 2002-249 and 2002-250, which the Canadian Aviation Advisory Council (CARAC) has approved, will be published in the near future. These NPAs amend CAR PartVI, making operators' responsibilities in this matter clear. Transport Canada will continue to monitor this investigation and is considering taking unilateral action through an airworthiness directive, should the Federal Aviation Authority not mandate MDHI Mandatory Service BulletinSB369D-201R1. MDHI no longer manufactures main-rotor blades for 500-series helicopters. Under parts manufacturing approval, Helicopter Technologies Corporation (HTC) now assures the supply of these blades. HTC has incorporated an ultrasonic inspection as part of its manufacturing process to check for subsurface defects, voids, and cracks that, if undetected, may affect the structural integrity of the blades.Safety Action The US Federal Aviation Authority is working on a notice of proposed rulemaking to expedite an airworthiness directive to effect mandatory compliance with MD Helicopters Inc.'s (MDHI) Service Bulletin SB369D-201R1 and Torque Events Inspection/Assessment. Transport Canada noted that compliance with publications recommending the incorporation of modifications, the performance of inspections, or times between overhaul, are optional. However, aircraft owners have a duty to be aware of the contents of these publications and to evaluate the need for compliance in light of their own circumstances. Commercial operators should have a formalized process for conducting this evaluation as part of the evaluation program required by Canadian Aviation Regulation706. Notice of Proposed Amendments (NPAs) 2002-249 and 2002-250, which the Canadian Aviation Advisory Council (CARAC) has approved, will be published in the near future. These NPAs amend CAR PartVI, making operators' responsibilities in this matter clear. Transport Canada will continue to monitor this investigation and is considering taking unilateral action through an airworthiness directive, should the Federal Aviation Authority not mandate MDHI Mandatory Service BulletinSB369D-201R1. MDHI no longer manufactures main-rotor blades for 500-series helicopters. Under parts manufacturing approval, Helicopter Technologies Corporation (HTC) now assures the supply of these blades. HTC has incorporated an ultrasonic inspection as part of its manufacturing process to check for subsurface defects, voids, and cracks that, if undetected, may affect the structural integrity of the blades.